The present invention relates to an apparatus and a method of decision-directed carrier recovery based on LMS method, which is appropriate for high speed satellite communication. In particular, the apparatus and the method in accordance with an embodiment of the present invention compensate frequency and phase of carrier by processing baseband signals at complex domain.
Generally, in the demodulator of a digital wireless communication system, the angular frequency and the phase offsets that are contained in samples to be decided to the symbol data should be compensated.
Carrier components that are included in received signals are removed by local oscillators at intermediate frequency conversion part after passing through radio frequency receiving part. Even though carrier components are removed by this process, practically carrier frequency offset and phase offset exist. The remaining frequency and phase are detected and compensated at baseband for simple implementation of demodulators.
Since high intermediate frequencies and radio frequencies are utilized in satellite links, the narrower the bandwidth of information bearing signal, the more the signal is affected in the course of frequency down-conversion.
Therefore, a frequency synthesizer is required to convert the signal to the baseband signal correctly. On the contrary, the frequency deviation via the satellite link for wideband signals such as the QPSK (Quadrature Phase Shift Keying) signal with a transmission speed of more than 45 Mbps, can be controlled within 5,000 particles per million (PPM) times the symbol rate by using relatively accurate oscillators for burst-mode communications.
Many studies have been performed in order to improve compensation capability of carrier. In 1983, Viterbi and Viterbi proposed xe2x80x9cnonlinear estimation of PSK modulation carrier phase with application to burst digital communication.xe2x80x9d, IEEE transactions on information theory, Vol. IT-32. But the algorithm proposed by Viterbi and Viterbi is disadvantageous in that noise gets significantly increased as M of the algorithm is increased.
In 1991, Fitz analyzed the equivocation problem in Viterbi and Viterbi scheme caused by M-th powering and suggested a solution for it at M. P. Fitz, xe2x80x9cEquivocation in nonlinear digital carrier synchronizersxe2x80x9d, IEEE transactions on Communications, Vol. COM-39, No. 11.
However, the algorithm of Viterbi and Viterbi has a drawback of increasing noise levels excessively compared to the signal as M increases.
F. Classen, H. Meyer, and P. Sehier proposed estimators for frequency and phase by the decision-directed (DD) method and analyzed their performance at xe2x80x9cIn all feedforward synchronization unit for digital radioxe2x80x9d, Proceedings of VTC""93. Through this method doesn""t employ VCO (voltage controlled oscillator). Though the scheme does not require any VCO, Read-Only Memory (ROM) tables are required in order to correspond the estimated phases to the complex values,
In addition, Fitz suggested a decision-directed carrier synchronization technique for burst-mode modems, which can be applicable to TDMA (Time Division Multiple Access) at xe2x80x9cDecision-directed burst-mode carrier synchronization techniques"", IEEE transactions on communications, Vol. COM-40, No. 10, October 1992. In the scheme, the estimates for frequency require a divider instead of a VCO. Therefore, though it has a wide capture range of frequency, the calculation of division renders it unsuitable for high-speed transmissions.
In addition, though the scheme suggested by Classen and Fitz has fast acquisition performance, two determined symbols are used for frequency detection, which means one symbol error affects two continuous frequency detection and performance is deteriorated at low SNR (Signal to Noise Ratio).
M. S. Kim proposed a decision-directed carrier recovery scheme for high-speed satellite communications in burst modes at xe2x80x9cDesign and analysis of decision-directed carrier recovery for high-speed satellite communicationsxe2x80x9d, IEICE transactions on communications, Vol. E81-B, No. 12, December 1998. The scheme doesn""t require any divider or VCO and uses only a decided symbol in obtaining frequency and phase estimates. In addition, regarding high-speed data communications, the scheme has 5,000 PPM [Hz/symbol] in the frequency capture range. Following equation 1 illustrates AR (autoregressive)-AR (autoregressive) model frequency estimator, phase estimator, and total estimator.   "AutoLeftMatch"                                                                                                              Ω                    ^                                                        AR                    ,                    k                                                  =                                  xe2x80x83                                ⁢                                                                            β                      ⁢                                              xe2x80x83                                            ⁢                                                                        Ω                          ^                                                                          AR                          ,                                                      k                            -                            1                                                                                                                +                                                                  (                                                  1                          -                          β                                                )                                            ⁢                                                                        ⅇ                          ^                                                                          ω                          ,                          k                                                                                                      ≅                                                            ⅇ                                              j                        ⁡                                                  (                                                                                    k                              ⁢                                                              xe2x80x83                                                            ⁢                                                              ω                                0                                                                                      +                                                          θ                              0                                                                                )                                                                                      ⁢                                                                  (                                                  1                          -                          β                                                )                                                                    1                        -                                                  β                                                      j                            ⁢                                                          xe2x80x83                                                        ⁢                                                          ω                              0                                                                                                                                                                                                                                                                                        Φ                    ^                                                        AR                    ,                    k                                                  =                                  xe2x80x83                                ⁢                                                                            γ                      ⁢                                              xe2x80x83                                            ⁢                                                                        Φ                          ^                                                                          AR                          ,                                                      k                            -                            1                                                                                                                +                                                                  (                                                  1                          -                          γ                                                )                                            ⁢                                                                        ⅇ                          ^                                                                          θ                          ,                          k                                                                                                      ≅                                                            ⅇ                                              j                        ⁢                                                  xe2x80x83                                                ⁢                                                  ω                          0                                                                                      ⁢                                                                  (                                                  1                          -                          β                                                )                                                                    1                        -                                                  β                          ⁢                                                      xe2x80x83                                                    ⁢                                                      ⅇ                                                          j                              ⁢                                                              xe2x80x83                                                            ⁢                                                              ω                                0                                                                                                                                                                                                                                                                                                                      R                    ^                                                        AR                    ,                                          k                      +                      1                                                                      ≡                                  xe2x80x83                                ⁢                                                                            Ω                      ^                                                              AR                      ,                      k                                                        ⁢                                                            Φ                      ^                                                              AR                      ,                      k                                                                      ≅                                  ⅇ                  ⁢                                                            "LeftBracketingBar"                                              1                        -                                                  β                                                      j                            ⁢                                                          xe2x80x83                                                        ⁢                                                          ω                              0                                                                                                                          "RightBracketingBar"                                        2                                                                                                            [Equation  1]                    
where, xcex2,xcex3 are positive numbers within the range (0,1) for controlling noise bandwidth and make influence on phase estimator, frequency estimator, and total estimator. This scheme has a drawback that the total estimator is too sensitive to the amplitude of the recovered signal as the amount of frequency offsets increases.
An apparatus and a method of decision-directed carrier recovery based on LMS method are provided.
The apparatus of decision-directed carrier recovery based on LMS method in accordance with an embodiment of the present invention includes frequency estimating means, phase estimating means, and complex number deciding means.
The frequency estimating means compensates a carrier frequency offset of an external input signal and recovers a carrier frequency of the input signal. The phase estimating means compensates a phase offset of an output signal outputted from the frequency estimating means and estimates a carrier phase of the frequency offset compensated signal. The complex number deciding means determines a complex number for an output signal outputted from the phase estimating means and provides the determined complex number to the frequency estimating means and the phase estimating means.
Preferably, the frequency estimating means includes first conjugate complex number generating means, first multiplying means, averaging means, second conjugate complex number generating means, and second multiplying means. The first conjugate complex number generating means receives the complex number from the complex number deciding means and generates first conjugate complex number. The first multiplying means multiplies the external input signal by the first conjugate complex number. The averaging means averages an output signal of the first multiplying means. The second conjugate complex number generating means receives the output of the averaging means and generates a second conjugate complex number. The second multiplying means multiplies the external input signal by the second conjugate complex number and generates a frequency offset compensated signal.
Preferably, the averaging means includes a low pass filter for averaging the output signal of the first multiplying means.
Preferably, the phase estimating means comprises includes third conjugate complex number generating means, third multiplying means, parameter generating means, fourth multiplying means, accumulating means, fourth conjugate complex number generating means, fifth multiplying means, and adding means. The third conjugate complex number generating means receives the complex number from the complex number deciding means and generates a third conjugate complex number. The third multiplying means multiplies the frequency offset compensated signal by the third conjugate complex number. The frequency offset compensated signal is output of the second multiplier of the frequency estimating means. The parameter generating means generates a control parameter. The fourth multiplying means multiplies an output of the third multiplying means by the control parameter. The accumulating means accumulates an output of the fourth multiplying means. The fourth conjugate complex number generating means receives an output of the accumulating means and generates a fourth conjugate complex number. The fifth multiplying means multiplies the fourth conjugate complex number by the frequency offset compensated signal. The adding means adds an output of the complex number deciding means to an output of the fifth multiplying means and generates an output of the addition to the third conjugate complex number generating means.
A method of decision-directed carrier recovery based on LMS method in accordance with an embodiment of the present invention includes a frequency estimating step, a carrier phase estimating step, and a complex number deciding step. The frequency estimating step compensates a carrier frequency offset of an external input signal and thereby recovers a carrier frequency of the input signal. The carrier phase estimating step compensates a phase offset of the frequency offset compensated signal and thereby recovers a carrier phase of the input signal. The complex number deciding step determines a complex number for the phase recovered signal and executes the frequency estimating step and the carrier phase estimating step.
Preferably, the frequency estimating step includes a first conjugate complex number generating step, a first multiplying step, an averaging step, a second conjugate, complex number generating step, and a second multiplying step. The first conjugate complex number generating step receives the complex number and generates a first conjugate complex number. The first multiplying step multiplies the external input signal by the first conjugate complex number and generates a first multiplied signal. The averaging step averages the first multiplied signal and generates an average signal. The second conjugate complex number generating step receives the average signal and generates a second conjugate complex number. The second multiplying step multiplies the external input signal by the second conjugate complex number and generates a second multiplied signal.
Preferably,the carrier phase estimating step includes a third conjugate complex number generating step, a third multiplying step, a parameter generating step, a fourth multiplying step, an accumulating step, a fourth conjugate complex number generating step, a fifth multiplying step, and an adding step. The third conjugate complex number generating step receives the complex number and generates a third conjugate complex number. The third multiplying step multiplies the frequency offset compensated signal by the third conjugate complex number and generates a third multiplied signal. The parameter generating step generates a control parameter. The fourth multiplying step multiplies the third multiplied signal by the control parameter and generates a fourth multiplied signal. The accumulating step accumulates the fourth multiplied signal and generates an accumulated signal. The fourth conjugate complex number generating step receives the accumulated signal and generates a fourth conjugate complex number. The fifth multiplying step multiplies the fourth conjugate complex number by the frequency offset compensated signal and generates a fifth multiplied signal. The adding step adds the complex number to the fifth multiplied signal and returns to the third conjugate complex number generating step.